Stereochemistry is the area of chemistry concerned with the 3D arrangement of atoms in molecules. Understanding the structure of a compound and how its atoms are arranged in space is critical for the understanding of how it behaves.
In organic chemistry, a wedge-dash notation is commonly used to denote the orientation of atoms in a molecule. This uses dotted lines to represent a bond to an atom which is further away from the viewer, a wedge to represent a bond to an atom which is nearer to the viewer, and solid lines to show a bond in plane with the molecule.
In biochemistry there are two other common ways of drawing molecules: Fischer projections and Haworth projections. These projections are used for carbohydrates to visualise their 3D structure and for comparison between different carbohydrates.
Sugars are shown in their open chain form by Fischer projections. This is when they form a long chain of carbons in a line, rather than their cyclic form.
D-aldohexose sugars: allose, altrose, glucose, mannose, gulose, idose, galactose, talose. Fischer-like projections. Image Credit: molekuul_be / Shutterstock
The carbon atoms of the main chain in the carbohydrate molecule are connected vertically, while hydroxyl groups and hydrogen atoms are bonded horizontally.
Below is a Fischer projection of D-glucose in its open chain form.
Horizontal lines in this projection illustrate bonds that come out of the page, whilst the vertical lines show bonds that are in the page.
The main advantage of Fischer projections is the ease of visually identifying stereochemical properties of a carbohydrate and being able to compare the difference between different carbohydrates quickly and easily. For example, it is very easy to tell the difference between two enantiomers; they are molecules that are a mirror image of each other.
A Fischer projection can be rotated by 180 degrees without changing the molecule’s stereoisomerism, but it cannot be rotated by 90 degrees or a different enantiomer will be illustrated. Care therefore needs to be taken when using a Fischer projection to represent a carbohydrate, as small changes can affect the molecules characteristics.
Haworth projections are used to draw the structure of carbohydrates in their cyclic form.
D-aldohexose sugars: allose, altrose, glucose, mannose, gulose, idose, galactose, talose. Haworth-like projections. Image Credit: molekuul_be / Shutterstock
Many carbohydrates are naturally found in their cyclic form or are a polymer of many rings joined together. For example, cellulose, which gives plant cells their strength and rigidity, is a polymer of many beta-glucose rings.
Below is the Haworth projection for the cyclic form of D-glucose.
Thicker lines on these projections represent bonds between atoms that are closer to the viewer. As we can see from the above diagram, carbons 2 and 3, as well as their respective hydroxyl groups, are closer to the viewer than the other carbons.
Although Haworth projections are a useful way to depict cyclic sugars, they are not entirely accurate in representing the position of all the atoms in space. There is a way to draw cyclic carbohydrates called the “chair” conformation, which depicts the layout of atoms more accurately. However, the chair conformation can make it more difficult to determine basic stereochemistry of the carbohydrate.
Fischer projections were developed by Emil Fischer, who won the second Nobel Prize in Chemistry in 1902 for his work with carbohydrates and finding important links between biology and chemistry. Fischer completely changed our view on carbohydrates, and by studying certain natural fruit sugars, he could establish a simple but very useful way to depict them. This was named the Fischer projection.
Norman Haworth expanded on the research of Fischer, to further characterize many more carbohydrates. Throughout the First World War, Haworth organized laboratories producing drugs and fine chemicals, but it was after the war that he developed his new format for depicting carbohydrate structure.
Haworth went on to be awarded the Nobel Prize in Chemistry in 1937 for his investigations of carbohydrates and vitamin C. The Haworth projection is still commonly used today for depictions of carbohydrates in organic chemistry and biochemistry.